Femtosecond investigation of electron thermalization in gold

Electron-electron and electron-phonon thermalization processes are investigated in gold films using a high-sensitivity multiple-wavelength femtosecond pump-probe technique. A nonequilibrium electron distribution is excited by free-carrier absorption of an infrared femtosecond pulse and its relaxation dynamics followed by measuring the transient reflectivity and transmissivity with a visible probe pulse. Measurements performed in a thin gold film in the very low perturbation limit (Δ${\mathit{T}}_{\mathit{e}}$∼20 K) yield evidence for the existence of a non-Fermi electron distribution with an electron thermalization time of ∼500 fs and an electron-lattice interaction time of 1 ps. The apparent thermalization dynamics of the electron gas is much faster in optically thick samples and is shown to be dominated by transport effects.